1. Field of the invention
The present invention relates to a microwave-powered plasma-generating apparatus and method and, more particularly, is applicable to an ion source suitable for use in an ion milling apparatus or a reactive ion beam etching apparatus and to a plasma apparatus suited for use in a plasma etching process or other plasma process such as plasma CVD.
2. Description of Related Art
An increasing use of plasma-generating apparatus in recent years has been to provide ion beams for ion milling apparatus and reactive ion beam etching apparatus. The variety of design and the high performance of semiconductor integrated circuits and other thin film devices developed recently have led to to use of a fine treatment etching method using a reactive ion beam which has good directional characteristics, in place of the previous etching method using a plasma. Such ion beams are also used in ion milling, in which a non-reactive ionic species, e.g. argon ions, are employed.
To provide an ion source for such ion beam etching or milling apparatus, in the conventional technology there has generally been used a method of generating ions by DC discharge while supplying thermoelectrons from a filament. Such an ion source has the problem that the filament is rapidly consumed, if it is used to produce an ion beam from highly reactive gases such as oxygen or chlorine.
To solve this problem, there has been developed an ion source which does not use any filament, but in which typically a plasma of a suitable gas is established by introducing microwave radiation into a magnetic field generated by a solenoid coil. The ions are extracted from the plasma by an ion extracting electrode.
As an example of this microwave-powered ion source, reference is made to IEEE Transactions on Nuclear Science, vol. NS-26, No. 2, April 1979, pages 2120-2127. One of the apparatuses illustrated is shown in FIG. 1 of the drawings accompanying the present application. Around a discharge chamber 201 are arranged several solenoid coils 202 and a permanent magnet hexapole 203. Although this magnet hexapole is not described in detail, it apparently has the structure shown in FIG. 1(b). Microwave radiation is fed in by a waveguide 207 through a window or port 206, and a gas or plasma of low concentration is supplied through a gas introduction port 213. Plasma is generated in the chamber 201 and ions are extracted into a further vacuum chamber 210 by an ion extracting electrode 215. Vacuum connections are shown at 211 and 212.
The ions established by this apparatus in the plasma, as electrons at higher velocities bombard ions of lower valency, are multivalent. The electrons are confined by the multipolar magnetic field 230 established by the magnet hexapole 203. It is the aim of this disclosure to achieve this production of higher valency ions. As a result of the confinement by the multipolar magnetic field, the number of collisions of the electrons with the ions of lower valency is increased, so that more multivalent ions can be produced than with a structure composed of the solenoid coils only.
FIG. 2 of the accompanying drawings shows another conventional technology published in JP-A-63-279599. Plasma is formed in a vacuum chamber 110 from gas introduced via a port 111. The substrate 117 to be treated is mounted on a holder 116 in the chamber 110. Around the chamber 110 are a solenoid coil 113 and a plurality of permanent magnets having polarities as indicated in FIG. 2. It is stated that an axial magnetic field of about 875 Gauss (0.0875 Tesla) is applied in the chamber 100 by the permanent magnets 118, while microwave radiation is introduced through a window 115 from a waveguide 114. Although the figure in JP-A-63-279599 suggests that the magnetic field generated is as shown on the right-hand side of FIG. 2, the present applicants believe that the field is better represented as at the left-hand side of FIG. 2, which shows an axial field.
The method of operation of the apparatus described in JP-A-63-279599 is to superpose a magnetic field by means of the coil 113 (which is located axially above the magnet 118), until discharge is initiated, after which the current of the coil 113 is cut off.
Another document showing permanent magnets employed axially spaced from a solenoid coil, in a microwave plasma-generator, is U.S. Pat. No. 4,778,561. Specifically, a ring of permanent magnets is arranged around the plasma chamber, at some axial distance from the coil, with each magnet radially directed with a south pole close to the chamber and its north pole remote from the chamber. The aim is to create a more uniform magnetic field at the transverse plane of the permanent magnets, due to the summation of the field produced by the coil and the magnets at that region. It is also stated to be an aim that the magnetic field at the extraction system, which removes ions from the plasma-generation region, is minimal. How this is actually achieved is not clear from this disclosure.
Reference to also made to JP-A-2-123640 which shows a microwave-powered plasma-generating apparatus in which either a conventional solenoid coil or a ring of permanent magnets with their north pole-south pole directions aligned in the axial direction of the apparatus is employed to provide the magnetic field for generation of the plasma. The solenoid coil if provided is located outside the chamber, while the assembly of magnets is inside the vacuum chamber. The wall of the vacuum chamber contains a large number of small permanent magnets arranged with their poles of the same kind opposed to each other, to provide a multipolar magnetic field in the vicinity of the chamber walls. The flux of this multipolar field is indicated as close to the chamber walls and not extending across the chamber to the central region of the microwave window where the plasma is generated.
The problem with the above conventional technology is no attention is directed to control the leakage of magnetic flux in such microwave plasma generators, in order that the magnetic field strength at the region of ion extraction from the plasma, or alternatively at the region of a substrate treated by the plasma, is as low as possible. Curve 101 of FIG. 4 of the accompanying drawings discussed more below indicates how the axial component of magnetic flux density of a solenoid coil varies with axial position. The effect of the solenoid field which remains at the ion extraction zone is to cause dispersion of the extracted ion beam. Moreover, when the ion beam is used in ion milling or reactive ion beam etching or when the plasma is used in plasma etching, a magnetic field which may be of the order of 300 Gauss (0.03 Tesla) may be applied to the workpiece, which undesirably may change the magnetic characteristics of the workpiece, or may have the result that a magnetic material cannot be used as the workpiece. With the increasing desire for magnetic fields of higher strength at the plasma-generating zone, this problem increases.